1. Field of the Invention
The present invention relates to a vibration isolator for use in a general industrial machine or the like and adapted to absorb and attenuate vibrations from a vibration-generating portion.
2. Description of the Related Art:
In an automobile engine, a vibration isolator serving as an engine mount is interposed between the engine and the chassis, so as to prevent the vibrations of the engine from being imparted to the chassis.
Vibrations occurring in the engine include so-called shaking vibrations which occur when the vehicle travels at high speed, as well as idling vibrations which occur during idling and when the vehicle travels at about five kilometers per hour.
In general, the shaking vibrations have a frequency of less than 15 Hz, while the idling vibrations have a frequency of 20 to 40 Hz, so that the frequency differs between the shaking vibrations and the idling vibrations.
A vibration isolator of a liquid-sealed type has been proposed as a vibration isolator for absorbing the shaking vibrations and the idling vibrations. This vibration isolator is provided with a main liquid chamber and an auxiliary liquid chamber, and the main liquid chamber and the auxiliary liquid chamber are connected to each other via a plurality of limiting passages of different sizes. Each of the limiting passages is opened or closed by a changeover valve which is driven by a driver such as a motor. Desired limiting passages connect the main liquid chamber and the auxiliary chamber, and a liquid passes through the connecting limiting passages and is thereby subjected to passage resistance or liquid-column resonance, thereby absorbing the vibrations of desired frequencies.
With this vibration isolator, however, the changeover valve is provided in the interior of the vibration isolator, while the driver such as the motor is disposed on the outside of the vibration isolator. Accordingly, the interior of the vibration isolator becomes complicated owing to component parts including the changeover valve. In addition, a hole for allowing the interior and the exterior of the vibration isolator to communicate with each other is provided to connect the driver such as the motor to the changeover valve. Through this hole, the changeover valve is connected to a driving shaft of the motor or the like. A seal is provided in a gap between the hole and the driving shaft to ensure that the liquid will not leak through the gap. Since the driving shaft and the seal undergo sliding relative to each other, the durability becomes an important problem. Hence, if emphasis is placed on the durability, the structure of the seal portion becomes complicated.